During a typical operation requiring extracorporeal circulation, blood from the cardiovascular system of the patient is taken from the patient and collected in a container such as a venous reservoir. Typically, a pump is used to withdraw the blood from the venous reservoir and then deliver it to a gas exchanger, such as an oxygenator, which serves as an external lung, as it exposes the blood to the appropriate percentage of oxygen. The gas exchanger, which may have more than one outlet, may be combined with a heat exchanger to also control the temperature of the blood. The blood is passed from one outlet of the gas exchanger and delivered to a blood filter, which removes gaseous microemboli, fat emboli, aggregates and microaggregates, and other debris. During some extracorporeal protocols, the blood filter comprises a filter assembly that also removes leukocytes from the blood. From the filter, the blood is usually returned via the arterial line directly to the vascular system of the patient.
Ancillary circuits, typically including one or more additional pumps and a small reservoir, may be used to salvage blood from the operative site. The salvaged blood is delivered to a cardiotomy reservoir where it can be stored until the surgeon returns the blood directly or indirectly through the gas exchanger to the patient's cardiovascular system. During some protocols, the blood is directed from the cardiotomy reservoir to the venous reservoir, and returned to the patient via the arterial line as described previously. Another circuit, the cardioplegia circuit, may be used so that blood exiting another outlet of the oxygenator can be treated with chemical compounds and/or cooled, and delivered to the heart, to arrest the contractions of the myocardium. At the appropriate time, the blood is treated with a less concentrated chemical compound and/or warmed, and delivered to the heart before normal contractions are re-instituted. By these protocols, the collected blood is salvaged and the need for supplemental blood replacement may be minimized.
As noted earlier, some protocols involving the extracorporeal circulation of blood provide for some leukocyte depletion of the blood. Leukocyte depletion may be desirable as leukocytes can be activated during extracorporeal circulation, leading to damage as these activated leukocytes contact internal organs, particularly ischemic tissues, i.e., tissues in which no blood is flowing such as the heart and lungs during certain surgical procedures. Moreover, exposure to the most common leukocyte, the granulocytic neutrophil, is undesirable, since this leukocyte has been implicated as the mediator of tissue destructive events in a variety of disorders, including, for example, reperfusion injury. Additionally, circulating leukocytes have been implicated in a variety of other undesirable effects, including, but not limited to, transfusion complications, and pulmonary edema.
However, while some protocols involving extracorporeal circulation may provide for blood filtration, and even provide for leukocyte depletion, these protocols may suffer from several drawbacks. For example, since the blood filter can be in the arterial line leading directly to the patient, a blockage in the filter could lead to an interruption in blood flow, which is undesirable. Accordingly, some protocols provide for bypassing the filter. However, while bypassing the filter allows blood flow back to the patient, the use of the bypass means that the undesirable material can no longer be removed from the blood by the filter.
Moreover, a surgeon may prefer to utilize a filter that provides for relatively inefficient removal of undesirable material to minimize the risk of blocking. Illustratively, since a more efficient filter, e.g., including at least one of finer fibers, increased surface area, smaller pores, and a more dense filter element, may exhibit an undesirably high pressure differential between the inlet and the outlet as the undesirable material accumulates in the filter, the surgeon may prefer to utilize a less efficient filter.
Additionally or alternatively, since some filter assemblies, especially leukocyte filters assemblies, may have larger hold up volumes, some surgeons may prefer to avoid "tying up" the larger volume of blood in the line leading directly to the patient. This tying up of a large volume of fluid may be undesirable when blood needs to be quickly returned to the patient.
Moreover, since some extracorporeal treatment protocols may include passing blood through the filter for several hours, e.g., from about 2 hours or less to about 10 hours or more, the fear of accumulating undesirable material over this indeterminate amount of time may lead some surgeons to err on the side of caution by utilizing a less efficient filter, rather than face the unacceptably high risk of clogging.
Additionally, typical extracorporeal treatment protocols fail to allow the surgeon to optimize blood flow rates and/or blood flow paths for different purposes, since so much of the blood is usually flowing through the filter and back to the patient. In some embodiments, it may be desirable to provide one rate and/or flow path for more efficient leukocyte depletion, and another rate and/or flow path for the optimum return of oxygenated blood to the patient. It may be desirable to recirculate blood through a leukocyte depletion filter assembly in a section of the circuit other than the section of the circuit leading directly back to the patient, so that some of the blood reaching the patient has even fewer leukocytes. Additionally, or alternatively, it may be desirable to provide leukocyte depletion at a particular point during the operation, e.g., before reperfusion; or to provide it during a plurality of points, rather than providing leukocyte depletion during the entire operation.
Accordingly, there is an unmet need in the art for methods, systems, and devices for providing increased efficiency in removing undesirable material, particularly leukocytes, from biological fluid in an extracorporeal circuit. Moreover, there is a need in the art for methods, systems, and devices for providing efficient removal of undesirable material from biological fluid in an extracorporeal circuit while minimizing the risk that the flow of biological fluid back to the patient could be compromised.
The present invention provides for ameliorating at least some of the disadvantages of the prior art. These and other advantages of the present invention will be apparent from the description as set forth below.